Flame stoichiometries

This experiment describes a fractional factorial design used to examine the effects of flame height, flame stoichiometry, acetic acid, lamp current, wavelength, and slit width on the flame atomic absorbance obtained using a solution of 2.00-ppm Ag+. 

In order to probe some of these questions - an essential endeavor in forming a clear interpretation of our results - we wish to compare our experimentally-determined data with predictions from a simple model. The experimental data available (See Fig. 3) are instantaneous values of flame temperature from the N2 Stokes/anti-Stokes intensity ratio (plotted as histograms in Fig. 4) and simultaneously-obtained values of Nj density (determined from the absolute value of the N. Stokes intensity calibrated against the value obtained for N2 in ambient air). Accordingly, we have produced "comparison" plots using the following scheme (24) If we calculate flame gas density and temperature as a function of flame stoichiometry (i.e., as a function of the fuel/air equivalence ratio see Fig.7), then we can... 

Figure 7. Plots of major flame species and temperature for Fit-air flame as a function of flame stoichiometry (i.e., fuel-air equivalence ratio ) for adiabatic conditions...

For Cr analysis only, flame stoichiometry at the was stoichiometric and at the high level was rich low level... 

Flame stoichiometry appears to be significant for the analysis of Cr in samples prepared by the one-step HNO3 digestion. Variables which are significant when data are obtained by using a rich air-acetylene flame are found not to be significant when a lean air-acetylene flame is used for analysis. 

Be, Cd, and Pb showed no flame stoichiometry effects Linearity of curve less than 0.997... 

The comparison of single analyte standard curves with mixed analyte standard curves was repeated, adding La flame buffer to all standard solutions. The standard matrix was 0.5 percent La, 10 percent HNO3. These results are presented in Table XV. The only slope ratios which differ from unity by more than one percent are Pb analyzed in a lean flame and Cu analyzed in a lean or rich flame. Therefore, when the analysis of a sample depends on the choice of calibration standards and flame stoichiometry, a La flame buffer added to both samples and standards alleviates the dependence. 

Figure 1. The effect of the acetylene-to-air ratio on the calibration curve of nickel. Curve 1 was obtained with a fuel-rich fame stoichiometry Curves 2 and 3 were obtained with progressively less fuel-rich flame stoichiometries. All experiments were performed with single analyte solution in 10% HNOa.

The optimization of the atomic absorption method of determining metals in particulates found in the air of workplace is described. The Plackett-Burman Youden-Steiner balanced incomplete block designs as well as single-factor experiments were utilized with ten metals Be, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, and Pd. Of the parameters tested, perchloric acid digestion, flame-stoichiometry, and the composition of the calibration standards were the most significant. Perchloric acid affected the recoveries of chromium. This was attributed to the formation of volatile chromylchloride. Flame-related phenomena and interelemental effects were brought under control using lanthanum flame buffer. 

The fuel and oxidant mixture must be controlled to provide the proper flame conditions for the element being analysed. A modem spectrometer should have a gas control system providing the precise and safe regulation which is important if reproducible results are to be obtained, particularly for those elements that show great dependence on flame stoichiometry. 

With the 309.27/309.28 doublet, sensitivity depends on spectral bandpass a narrow 0.2 nm bandpass is recommended to minimise intense emission of the flame. Absorbance depends critically on flame stoichiometry and observation height. S/N can be improved by increasing lamp current and optimizing fuel flow. 

A narrow spectral bandpass of ca. 0.2 nm is required with the 357.87 chromium line to eliminate nearby 357.66 nm and 358.23 nm argon lines when an argon-filled light source is used. In an air—acetylene flame, sensitivity and chemical interferences are critically dependent on flame stoichiometry and observation height. 

Absorbance is highly dependent on lamp current and flame stoichiometry. Hollow-cathode lamps must be operated at fairly low currents to prevent self-absorption good lamp stability, however, permits use of high instrumental scale expansion. The spectral bandpass used is not critical. For aqueous solutions very few chemical interferences have been reported in the air—acetylene flame depression of Cd absorbance is caused by large amounts of silicon. 

Handbooks supplied by manufacturers contain valuable information for selecting instrument settings. However, in order to optimize the analyte s signal, burners should be clean and flame stoichiometry and burner position should be adjusted before each group of measurements is taken on standards and samples. 

Figure 2.3 shows a plot of the adiabatic equilibrium flame temperature for an air/CH4 flame and an 02/CH4 flame, as functions of the flame stoichiometry. There are several things to notice. The flame temperature for the air/CH4 flame is very dependent on the stoichiometry. For the 02/CH4 flame, the temperature is very dependent on the stoichiometry only under fuel-rich conditions. The temperature is not very dependent on the stoichiometry when the 02/CH4 flame is fuel lean. 

Instruments can be calibrated by preparing standard solutions over the concentration range of interest and measuring the absorption or emission of these under the same conditions as sample measurement. At least one standard should be run with each set of samples to determine any correction that should be applied to the calibration curve, because the variables of flame stoichiometry, aspiration rate, and positioning of the burner are difficult to reproduce precisely. 

Addition of chemically active compounds to flame, which are able to change the flame velocity, the flame propagation limits and the other macrokinetic parameters, seems to be the most effective way to control combustion. Of sp>ecial interest are chemically active inhibitors producing a noticeable effect on flame at low concentrations, which do not change the flame stoichiometry. Thousands of elementary reactions involving hundreds of sprecies proceed in hydrocarbon flame. However, the key reactions are those involving atoms and free radicals with their reaction rates being much faster than those of the other reactions. The inhibitors mainly interact and affect the above processes. 

According to (Shvartsberg et al., 2010) the variation of net rate of production/consumption of the chain carriers in the Fe-involving reactions versus < ) is mainly associated with a variation of ICS composition and variation of concentration of the chain carriers in the reaction zone of the flames with flame stoichiometry. The change of the flame temperature with ( ) plays minor role in variation of the rate of production of the active flame species. It may be explained by relatively low activation energies of the Fe-involving reactions from the mechanism (Linteris et al., 2000a), which do not exceed 6500 J/mol. 

The effectiveness of a certain inhibitor depends on features of its combustion chemistry, flame stoichiometry and conditions (pressure). These developments should be necessarily taken into consideration in the case of practical application of an inhibitor. 

---